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Old 07-16-2002, 06:49 PM
Posts: n/a
Mystery black box behind battery in 300E

Yesterday, I was rooting around the engine compartment to look for the resistor that retards the ignition by 6 degrees (found it eventually) but in the process found a box that I have no idea what is for. It is behind the cheezy black cover behind the battery. It is a black box that looks kinda like the DME unit, but has a knob on the top of it that can be selected to position "0" or "1". Right now it is on "1". What is this box? Thanks -- Mikey
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Old 07-16-2002, 08:04 PM
it leaks, its german
Join Date: Mar 2002
Location: raleigh nc
Posts: 1,111
Apparently you have a 90 or later production car with a MAS relay. The knob "locks" the relay to the connector.

Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.
Raleigh NC
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Old 07-17-2002, 04:19 PM
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Join Date: Jun 1999
Location: Raleigh, NC, US
Posts: 47
what's a MAS relay and why the differant positions (0,1)?
1996 S320
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Old 07-17-2002, 04:47 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
MAS is the combination relay that handles the heavy current users of the engine: fuel pump, A/C clutch, air pump clutch maybe others.

The two positions are connected and disconnected. The thing is so big that I guess MB decided there needed to be a lock to keep the thing engaged. If you turn the knob, the relay will rise from its position and disengage itself. It screws itself back in also (if you have everything lined up).
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 07-19-2002, 09:10 AM
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Join Date: Feb 2002
Location: Belgium,Europ
Posts: 196
Black box

My car is a W124 300TD turbo from sep. 1990 and in the compartiment behind
the battery there is a "black box" to. The manual mentions it as the "brains"
of the ABS brake system. They talk about it because it is important to take it out or deconnect when doing electric welding on the body: it wan't survive
and it is certainly VERY pricely!
Danny M.
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Old 07-19-2002, 06:52 PM
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Join Date: Jan 2002
Location: England
Posts: 1,841
The ABS Control unit (ECU) is also behind the battery, but usually has a very large multi-pin connector along the top, and no 1/0 switch.
I've never seen one of these MAS relay boxes before though, and would've been pretty confused by it!

2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black 2nd owner
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